Start-up system for forced flow vapor generator



Jan. 9, 1968 A. H. RUDD 3,362,164

START-UP SYSTEM FOR FORCED FLOW VAPOR GENERATOR Filed Oct. 4, 1965 58 k 33 PX 65 l 34 3e 12 11 1o -wvvw INVENTOR.

Alexander H. Rudd United States Patent 3,362,164 START-UP SYSTEM FOR FORCED FLOW VAPOR GENERATOR Alexander H. Rudd, Akron, Ohio, assignor to The Babcock & Wilcox Company, New York, N.Y., a corporation of New Jersey Filed Oct. 4, 1965, Ser. No. 492,464 11 Claims. (Cl. 60-105) This invention relates in general to a power plant system having a turbine arranged to be supplied with vapor from a forced circulation vapor generator and more particularly to apparatus for and a method of starting up such a system.

The general object of the present invention is the provision of a starting system of the character described so constructed and arranged as to simplify starting procedure; to provide low cost, rapid, controlled start-ups; to provide adequate protection of the turbine and vapor superheating section of the vapor generator to the end that thermal stresses on this equipment are within acceptable limits at all times; to provide maximum heat recovery during starting up and low load operation; and to permit matching of the vapor temperature to turbine metal temperature on hot restarts and a minimum differential of vapor temperature and turbine metal temperature on cold starts.

More specifically, the invention is directed to improvements in the construction and operation of a startup system of the type described in US. Patent No. 2,989,- 038 in which the discharge from the vapor generating section of a once-through boiler is passed into a flash tank, water from the flash tank is conducted to the inlet end of the vapor generating section of the boiler, vapor from the flash tank is passed through the vapor superheating section of the boiler and then condensed for return to the vapor generating section, and provisions are made for lay-passing the flash tank when the working medium is properly conditioned for rolling and loading the turbine.

A disadvantage of this system, as well as other prior start-up systems, has been erratic control of steam temperature due to two sequential operations during start-up and loading resulting in large changes in the heat storage level of the low temperature or primary section of the superheater which could not be easily measured because of exposure of the primary superheater to flue gases. Such changes in heat storage of the primary superheater either added or subtracted heat from the working fluid so that a change in the heat input from firing was required to control working fluid heat absorption and thus final steam temperature. But because the changes in heat storage could not be accurately determined, the firing rate could not be properly compensated, and erratic final steam temperatures resulted which could cause severe thermal stresses in steam piping and turbine parts.

The first operation causing severe heat storage change in the primary superheater involved a constant pressure change in the enthalpy of the working fluid entering the primary superheater. During turbine rolling and initial loading, saturated vapor (at approximately 1200 B.t.u./lb. enthalpy) was supplied from the flash tank to the inlet of the primary superheater, so that most of the heating surface of the primary superheater was well above satura tion temperature. However, further load increase required admission to the primary superheater of working fluid direct from the vapor generating section of the boiler at an enthalpy of approximately 850 B.t.u./lb. and moisture content of 50%. The moisture of such vapor generating section outflow initially would be evaporated from the stored heat in the primary superheater tube metal, but when this heat was exhausted, the moisture had the effect of attemperating the working fluid and rapidly reducing the steam temperature leaving the primary superheater.

The second operation causing heat storage change in the primary superheater was an increase of operating pressure of the primary superheater simultaneously with increase in load. On a typical installation the increase would be from 7% of full load flow and 1,000 p.s.i.g. to 33% of full load flow and 3,500 p.s.i.g. Such an increase in pressure required increasing the operating temperature of the primary superheater metals from about 545 F. (saturation at 1,000 p.s.i.g.) to about 725 F. (pseudosaturation temperature at 3,500 p.s.i.g.). It also involved increasing the weight of fluid stored in the constant volume of the primary superheater because of the approximately 3 to 1 change in specific volume with pressure increase. As described earlier, such changes in heat storage, because they are extremely difficult to measure, have resulted in erroneous firing rates and poor steam temperature control.

In accordance with the present invention, the abovedescribed start-up deficiencies are eliminated by special start-up provisions for utilizing superheater surface as vapor generating surface while rolling and loading the turbine, for maintaining the through flow of such surface at full boiler pressure at all times, for directing the outflow of such surface to a flash tank to provide low pressure vapor to the turbine during the initial stages of startup, and for regulating the outflow of such surface so that its enthalpy will match that of the flash tank vapor to provide transition without temperature variations from the use of the low pressure flash tank vapor in turbine startup to the use of the high pressure outflow of such superheater surface.

As generators of the forced flow type are well-known in the art, I have shown in the drawing the elements of such a generator in rudimentary form to assist in understanding my invention. While the starting system of the invention is adapted for use in a forced flow vapor generating and superheating unit designed for the production of superheated vapor at pressures and temperatures below the critical pressure of 3206 psi. and the critical temperature of 705 F a unit of this general construction being described and claimed in US. Patent No. 3,125,- 995, issued Mar. 24, 1964, it will be understood that the invention starting system can also be advantageously used in a forced flow vapor generator designed for supercritical pressures and temperatures.

In the power plant system illustrated, feedwater is supplied by a boiler feed pump 10 through a conduit 11 to a high pressure feedwater heater 12, then passes through a conduit 13 to the circuitry of a vapor generator 14 of the once-through forced flow type having a series fluid flow path including an economizer 16, vapor generating section 17, and superheating section 18. superheating section 18 comprises a primary superheater 18A, connected by a conduit 19 for flow of fluid from vapor gnerating section 17, and a secondary superheater 18B, connected for serie flow of fluid from primary superheater 18A by a conduit 15 containing a stop valve 20 and for series flow of fluid to a high pressure turbine 22 by a conduit 23 containing a stop valve 24 and a control valve 26. Valve 24 is by-passed by a conduit 21 containing a stop valve 25. Fuel and air for combustion are admitted to the vapor generator 14 through conduits 27 and 28 respectively; and the gaseous products of combustion, after passing over the vapor generating and superheating surfaces and economizer, are discharged through an outlet, diagrammatically shown at 29. Valves 27A and 28A represent the customary regulating means for the fuel and air respectively.

During normal operation, the working medium passes successively through economizer 16, vapor generating section 17 and superheating section 18. The superheated vapor outflow of superheating section 18 passes through conduit 23 to the high pressure stage of vapor turbine 22 for expansion therein, with the vapor then exhausting through a conduit 30 to a main condenser 31, where it is. condensed at a low pressure for return to the feedwater system. From the condenser the condensate passes by way of a conduit 32 to a pump 33 from which it discharges. through a conduit 34 and low pressure feedwater heater 36 to a direct contact type deaerating heater 37 which. serves to boil the condensate to eliminate any entrained. oxygen. Condensate from the deaerator passes through a conduit 38 to the suction side of feed pump for return to the vapor generator.

In accordance with the invention, a special by-pass. system is provided around the superheaters 18A and 18B and around the turbine. This system is contructed and. arranged to obtain highest operational flexibility, to minimize heat losses, and to provide full thermal protection of superheater surface and the turbine; and is used for cold and hot start-ups, for low load operation and for shutdown or emergency trip of the vapor generator.

The primary superheater by-pass comprises a conduit 39 containing a pressure breakdown valve 41 and having one end connected and opening to conduit 19 and its opposite end connected and opening to a flash tank 42, which serves as a receiving vessel for the by-pass fluid and separates steam and water. The secondary superheater by-pass includes a conduit 55 containing a pressure breakdown valve 41A and having one end connected and opening to conduit and its opposite end connected and opening to conduit 39 at a position intermediate valve 41 and flash tank 42. Provisions are made for directing flash tank steam to secondary superheater 183 by means of a conduit 60 containing a control valve 50 and a stop and check valve 50A and having its inlet end connected to flash tank 42 and its discharge end connected to conduit 15 at a point intermediate valve and superheater 18B, Valve 20 is by-passed by a conduit 15A containing a pressure reducing valve 20A and having one end opening and connected to conduit 55 at a point upstream of valve 41A and its opposite end connected and opening to conduit 60 at a location downstream of valve 50.

Drainage from the flash tank is provided by a conduit 49 containing a deaerator water pegging valve 51 and extending between flash tank 42 and deaerator 37; by a branch conduit 52 containing a valve 53 and having one and connected to conduit 55 at a point upstream of valve 51 and its opposite end connected to condenser 31; and by a branch conduit 65 containing a valve 70 and having one end connected to conduit 52 at a point upstream of valve 53 and its opposite end connected to the shell side of heater 12.

Steam connections from the flash tank to the high pressure feedwater heater, the deaerator and the turbine seals provide steam for these components during start-up, thereby recovering heat for the cycle. Thus a conduit 54, controlled by a valve 56, extends between flash tank 42 and the shell side of heater 12; a conduit 57, having a valve 58, has one end connected to conduit 54 at a position upstream of valve 56 and its opposite end connected to deaerator 37; and a conduit 59, controlled by a valve 61, leads from the flash tank to the turbine seals. A steam line 62, containing a valve 63 and leading from flash tank 42 to condenser 31, acts as a flash tank over pressure control by allowing any excess steam to flow to condenser 31.

In a typical cold start-up of the power plant system illustrated, about one quarter full load water flow is established through the boiler feed pump 10 which takes suction from deaerator 37 and forces fluid successively through conduit 11, heater 12, conduit 13, economizer 16, vapor generating section 17, and conduit 39 to fiash tank 42. superheater stop valve 20, pressure reducing valve 20A, secondary superheater by-pass valve 41A, turbine stop valve 24, and turbine stop valve by-pass 25 are closed so that there is no flow through superheating section 18. Valve 41 is set to maintain full load pressure at the outlet of the vapor generating section throughout th start-up.

From the flash tank 42 the water flows through conduit 49 to deaerator 37 and then back to feed pump 10. Initially valve 51 is set to maintain flash tank water level below normal. Firing is then commenced and gas temperature entering superheater 18 is held to approximately l,000 F. As the enthalpy of the fluid entering the flash tank increases, more residual heat is introduced to the deaerator, causing deaerator and flash tank pressure to rise. When deaerator pressure reaches about 20 p.s.i.a., valve 51 starts to close and the flash tank level control valve opens to allow flash tank drains to flow to the shell side of heater 12 to minimize heat loss, with the heater drains then passing through a conduit 71 to condenser 31. Conduit 71 is controlled by a valve 72. After 20 p.s.i.a. is reached in the deaerator, valve 51 controls deaerator pressure and the flash tank level control valve 70 now controls flash tank water level. If the flash tank water level exceeds the normal water level, valve 53 opens to divert the excess water to the condenser. Valve 70 is also controlled to close on high water level in heater 12.

As the temperature of the water entering the flash tank increases, flash steam becomes available and pressure in the flash tank continues to increase until the opening pressure of the seal steam control valve 61 is reached at which time separated steam is directed through conduit 59 and valve 61 to seal the turbine. When the turbine is sealed, condenser vacuum can be pulled. As the enthalpy of the fluid continues to rise, the flash tank pressure increases until the opening pressure of the deaerator steam pegging valve 58 is reached, at which point flash tank pressure is about 75 p.s.i.a. and steam from the flash tank goes to the deaerator 37 by way of conduits 54 and 57 until the deaerator pressure reaches about 25 p.s.i.a. At this time valve 51 is closed, since it has a set point of 20 p.s.i.a. deaerator pressure, and valve 70 is open so that more of the flash tank water is diverted to the shell side of heater 12 and thence passes to condenser 31 by way of conduit 71. Thus feedwater temperature is increased and heat is recovered by the use of flash tank steam to deaerator 37 and flash tank drain water in the high-pressure heater 12. When the flash tank pressure is 50-80 p.s.i.a. above the pressure at which the deaerator is satisfied with the quantity of steam delivered, the high pressure feedwater heater steam control valve 56 opens so that excess steam is diverted to the shell side of heater 12. As the flash tank pressure continues to rise, heater 12 pressure will rise, thus increasing feedwater temperature further and recovering more heat.

While outflow of vapor generating section 17 is directed through conduit 39 to flash tank 42 in the initial phase of start-up, secondary superheater by-pass valve 41A is kept closed until the water temperature at the inlet to primary superheater by-pass valve 41 exceeds 300 F. By so controlling valve 41A, it is never subjected to the severely erosive service of pressure breakdown of cold water. When the water temperature at the inlet of valve 41 exceeds 300 F., valve 41A is opened to a predetermined minimum position so that a portion of the vapor generating section outflow is directed through primary superheater 18A and then through conduit 39A to conduit 39 for flow to flash tank 42 along with the fluid passing directly from vapor generating section 17 to conduit 39. Valve 41A is maintained in this minimum position until the valve inlet fluid temperature is such that its corresponding enthalpy is about 1,200 B.t.u./lb. After the fluid entering valve 41A is so conditioned, the valve opening is regulated to maintain such set point temperature, while valve 41 is regulated to hold full load pressure at the outlet of the primary superheater throughout the start-up. By way of examples, the set point tempera- -in primary superheater 18A.

The passage of Working fluid through primary superheater 18A and by-pass 39A to flash tank 42 results in the heat absorbed by the primary superheater passing to the flash tank. In prior systems, primary superheater outflow was successively directed through the secondary superheater and a turbine by-pass to the condenser so that primary superheater heat absorption did not get to the flash tank. By directing the heat absorption of the primary superheater to the flash tank where it can be recovered, the unit may be heated up faster with a given firing rate or heated in the same time with a lower firing rate. Either way there is a saving in total fuel expended for start-up compared to prior systems.

When flash tank pressure reaches approximately 300 p.s.i.g., valve 50 is opened to allow flash tank steam to flow through conduit 60, secondary superheater 18B and main steam conduit 23 to a valve-controlled drain line 66, thereby warming up the main steam conduit and the secondary superheater. Drain line 66 leads from above the seat of stop valve 25 and its discharge may be directed to condenser 31.

As firing continues, the flash tank pressure increases. When the pressure, temperature and flow rate of the steam from flash tank 42 reaches predetermined conditions suitable to the turbine, then steam flow is directed into turbine 22 and flow through drain line 66 is discontinued. At this time flash tank pressure will be about 500 p.s.i.g. Steam flow to the turbine is controlled with turbine stop valve 25, allowing steam admission to the turbine with valves 26 wide open, which results in uniform heating of valve bowls, valve chests, and turbine first stage shell area. Valve 25 is controlled to throttle the pressure of the steam down to a level sufficient to roll the turbine. Any excess steam in the flash tank is discharged to condenser 31 by way of conduit 62, with valve 63 being regulated by flash tank pressure. The amount of opening of valve 63 serves as an index of the quantity of steam available in the flash tank for increasing turbine speed or load.

Since the heat absorbed by primary superheater 18A during start-up serves to generate steam, much lower firing rates are required than on prior start-up systems Where it served to superheat steam. The start-up system of the invention is particularly adapted for use in a forced flow vapor generating and superheating unit of the type described in US. Patent No. 3,125,995, wherein the primary superheater is disposed in the convection gas pass and absorbs heat primarily by convection and the secondary superheater is located in the convection gas pass upstream gas-wise of the primary superheater. A primary superheater so arranged is particularly eflective in absorbing heat during start-up when high excess air, high flue gas tempering or high flue gas recirculation is used. In addition, the low firing rates result in low gas temperature at the secondary superheater and therefore loW steam temperatures for rolling a cold turbine. When the start-up system is used to start a hot turbine, the unit may be fired at a higher rate to obtain high steam temperatures for rolling the turbine, and the flash tank over-pressure line 62 will relieve excess steam to condenser 31.

At a predetermined point in the loading of the turbine, control of the turbine is transferred from valve 25 to valve 26. Thus when the turbine is loaded to approximately 8% of maximum continuous rating, it is normally changed to partial arc-admission by simultaneously opening valve 25 and closing valves 26. Now valve 24 can be opened wide and valve 25 closed. From this point on, it is desirable to further load the turbine with throttle valves 26 in a fixed position to maintain the throttling temperature drop across valves 26 nearly constant. Load 6 increase is accomplished by increasing steam pressure in secondary superheater 18B and main steam line 23.

The turbine is loaded to 8% of full load steam flow at the maximum operating pressure of the flash tank 42, this being about 1,000 p.s.i.-g. To increase secondary superheater pressure above flash tank pressure, steam must be directed through pressure reducing valve 20A. Accordingly, valve 20A is opened until the secondary superheater pressure is slightly above that of the flash tank at which time steam flow from the flash tank to secondary superheater 18B will cease and valve 50A will close on check action. At this time flow to the flash tank will constitute only the difference between start-up flow and that passing directly to secondary superheater 18B via pressure reducing valve 20A. During opening of valve 20A, valve 4-1A is controlled to close by a comparable amount, thus maintaining flow through the primary superheater 18A substantially constant. Valve 41A is controlled to maintain primary superheater outflow temperature at a set value such that its corresponding enthalpy is about the same as the enthalpy of the saturated vapor of the flash tank 42. Since the flow in primary superheater 18A is kept constant, and since the enthalpy of the steam entering secondary superheater 18B is maintained constant, there is an orderly and controlled transition from secondary superheater inflow from the flash tank to inflow directed from the primary superheater without temperature variations.

The next step in loading the turbine involves increasing the secondary superheater pressure to its full load value. As such pressure is increased with throttle valves 26 in fixed position, steam flow to the turbine increases proportionately. In practice, throttle valves 26 are set to provide a turbine load at maximum flash tank pressure such that the turbine load will be about 30% of full load with full load pressure and the same throttle valve setting. For example, with the flash tank operating at a maximum value of 1,000 p.s.i.g. during start-up, the throttle valves should be set to provide a turbine load of approxi mately 8% of full load value in order to provide a turbine load of 30% at a full load throttle pressure of 3,500 p.'s.i.g. with the same valve setting.

Pressurization of the secondary superheater 18B is accomplished by gradually opening valve 20A. Since valve 20A is handling all vapor at a high specific volume, and since the outflow of valve 20A is directed to the relatively small volume of secondary superheater 18B, the secondary superheater pressure increase in response to such valve opening is almost immediate. This pressure response can be used as an index to correct the position of the pressure reducing valve 20A. The result is an orderly and controlled pressurization of the secondary superheater 1 88 since firing rate, secondary superheater outflow, and the position of valve 20A can be closely paralleled. Throughout start-up the primary superheater 18A is at a constant pressure, so that its heat storage does not change. During pressurization of the secondary superheater and loading of the turbine, the primary superheater outlet temperature set point, which governs the setting of secondary superheater by-pass valve 41A, can be adjusted to follow the natural heat absorption characteristics of the primary superheater. In application, valve 41A will be fully closed at about 10% full turbine load above which the primary superheater becomes increasingly effective in superheating steam, while the generation of steam therein gradually decreases.

When valve 20A is wide open, valve 41 is closed and all flow goes directly to the superheater. Thus flows to and from the flash tank cease and the pressure in the flash tank and by-pass system decays. Normal turbine extraction flows will replace flash steam and drains flows for deaera'tion and feedwater heating. At this time, the high pressure superheater stop valve 20 can be opened wide and valve 20A closed.

During the period after the flash tank reaches its norrnal operating pressure, any steam separated in the flash tank in excess of what is required for turbine sealing, deaeration, feedwater heating and turbine operation is discharged through the flash tank over-pressure control valve 63 to the condenser. During a cold start, there is little or no steam flow to the condenser.

The operation sequence for hot restarts and intermediate starts is essentially the same as for a cold start, except that the firing rate is higher in order to obtain higher steam temperatures leaving the secondary superheater.

While in accordance with the provisions of the statutes I have illustrated and described herein the best form and mode of the invention now known to me, those skilled in the art will understand that changes may be made in the form of the apparatus disclosed without departing from the spirit of the invention covered by the claims, and that certain features of the invention may sometimes be used to advatnage without a corresponding use oi? other features.

What is claimed is: 1. The method of starting a power plant in which, during normal operation, a vaporizable fluid is successively passed through a first heating zone and a second heating zone to a vapor turbine, said method comprising passing vaporizable fluid at about turbine full throttle pressure through the first heating zone in indirect heat transfer relation with heating gases and at a predetermined rate substantially less than full load flow,

passing a substantial portion of the first heating zone outflow through the second heating zone in indirect heat transfer relation with the heating gases,

passing second heating zone outflow to the separating zone while reducing its pressure to cause part of the fluid to flash into vapor,

recirculating fluid through the first heating zone, second heating zone and separating zone to the first heating zone to increase the enthalpy of the second heating zone outflow to a predetermined value, while increasing separating zone fluid pressure to a prede- *termined value substantially less than turbine full throttle pressure,

passing vapor formed in the separating zone to the turbine to warm and roll the turbine,

maintaining pressure in the first heating zone and the second heating zone at about turbine full throttle pressure, while regulating flow of fluid from the second heating zone to the separating zone so that the enthalpy of the fluid discharging from the second heating zone is maintained at about said predeterrnined value,

gradually establishing equilibrium of fluid pressures between the second heating zone and the turbine by passing second heating zone outflow to the turbine at a rate providing gradual increase in flow to the turbine, while discontinuing vapor flow from the separating zone to the turbine, and

discontinuing fluid flow from the second heating zone 'to the separating zone.

2. The method of starting a power plant in which, during normal operation, a vaporizable fluid is successively passed through a first heating zone and a second heating zone to a vapor turbine, said method comprising passing vaporizable fluid at about turbine full throttle pressure through the first heating zone in indirect heat transfer relation with heating gases and at a predetermined rate substantially less than full load flow.

passing first heating zone outflow through the second heating zone in indirect heat transfer relation with the heating gases,

passing second heating zone outflow to the separating zone while reducing its pressure to cause part of the fluid to flash into vapor,

recirculating fluid through the first heating zone, second heating zone and separating zone to the first heating zone to increase the enthalpy of the second heating zone outflow to a predetermined value, while increasing separating ZOne fluid pressure to a predetermined value substantially less than turbine full throttle pressure,

passing vapor formed in the separating zone to the turbine to warm and roll the turbine,

maintaining pressure in the first heating zone and the second heating zone at about turbine full throttle pressure, while regulating flow of fluid from the second heating zone to the separating zone so that the enthalpy of the fluid discharging from the second heating zone is maintained at about said predetermined value, said enthalpy value being about the same as the enthalpy of the separating zone vapor outflow,

gradually establishing equilibrium of fluid pressures between the second heating zone and the turbine by passing second heating zone outflow to the turbine at a rate providing gradual increase in flow to the turbine, while discontinuing vapor flow from the separating zone to the turbine, and

discontinuing flow from the second heating zone to the separating zone. 3. The method of starting a power plant in which, during normal operation, a vaporizable fluid is successively passed through a first heating zone and a second heating zone to a vapor turbine, said method comprising passing vaporizable fluid at about turbine full throttle pressure through the first and second heating zones in indirect heat transfer relation with heating gases and at a rate substantially less than full load flow,

passing heating zone outflow to a separating zone while reducing its pressure to cause part of the fluid to flash into vapor,

recirculating fluid through the first heating zone, second heating zone and separating zone to the first heating zone to increase the enthalpy of the second heating zone outflow to a predetermined value, while increasing separating zone fluid pressure to a predetermined value substantially less than turbine full throttle pressure,

passing vapor formed in the separating zone to the turbine to warm and roll the turbine,

maintaining pressure in the first heating zone and the second heating zone at about turbine full throttle pressure, while regulating flow of fluid from the second heating zone to the separating zone so that the enthalpy of the fluid discharging from the second heating zone is maintained at about said predetermined value, and

gradually establishing equilibrium of fluid pressures between the second heating zone and the turbine by passing second heating zone outflow to the turbine at a rate providing gradual increase in flow to the turbine, while discontinuing vapor flow from the separating zone to the turbine and while decreasing flow from the second heating zone to the separating zone so that flow through the second heating zone remains substantially constant until second heating zone outflow to the separating zone is cut off.

4. The method of starting a power plant in which, during normal operation, a vaporizable fluid is successively passed through a first heating zone and a second heating 65 Zone to a vapor turbine, said method comprising passing a vaporizable fluid at about full load pressure through the first heating zone in indirect heat transfer relation with heating gases and at a predetermined rate substantially less than full load flow, passing a portion of the first heating zone outflow to a separating zone while reducing its pressure to cause part of the fluid to flash into vapor, passing the remainder of the first heating Zone outflow through the second heating zone in indirect heat transfer relation with the heating gases,

passing the second'superheating outflowto the separating zone while reducing its pressure to cause part of the fluid to flash into vapor,

recirculating fluid through the first heating zone, second heating zone and separating zone to the first heating zone to increase the enthalpy of the second heating zone outflow to a predetermined value, while increasing separating zone fluid pressure to a predetermined value substantially less than turbine full throttle pressure,

passing vapor formed in the separating zone to the turbine to warm and roll the turbine, maintaining pressure in the first heating zone and the second heating zone at about turbine full throttle pressure, While regulating flow of fluid from the second heating zone to the separatingzone so that the enthalpy of the fluid discharging from the second heating zone is maintained at about said predetermined value, said enthalpy value being about the same as the enthalpy of the separating zone vapor outflow, and

gradually establishing equilibrium of fluid pressures between the second heating zone and the turbine by passing second heating zone outflow to the turbine at a rate providing gradual increase in flow to the turbine, while discontinuing vapor flow from the separating'zone tothe turbine and while decreasing flow from the second heating zone to the separating zone so that flow through the second heating zone remains substantially constant until second heating zone outflow to the separating zone is cut off.

5. The method of startinga power plant in which, during normal operation, a vaporizable fluid is successively passed through a fluidtheating zone and a superheating zone to a vapor turbine, said method comprismg passing a vaporizable fluid'at about full load pressure through the heating zone and the superheating zone in indirect heat transfer relation with heating gases and at a predetermined rate substantially less than 40 full load flow,

passing the fluid so heated to a separating zone while reducing its pressure to cause part of the fluid to flash into vapor,

recirculating fluid through the fluid heating zone, superheating zone and separating zone to the fluid heating zone to increase the enthalpy of the superheating zone outflow to a predetermined value, while increasing separating zone fluid pressure to a predetermined value substantially less than turbine full throttle pressure,

passing vapor formed in the separating zone to the turbine to warm and roll the turbine,

maintaining pressure in the heating zone and the superheating zone at about turbine full throttle pressure, while regulating flow of fluid from the superheating zone to the separating zone so that the enthalpy of the fluid discharging from the superheating zone is maintained at about said predetermined value, and

gradually establishing equilibrium of fluid pressures between the superheating zone and the turbine by passing superheating zone outflow to the turbine at a rate providing gradual increase in flow to the turbine, while discontinuing vapor flow from the separating zone to the turbine and while decreasing flow from the superheating zone to the separating zone so that flow through the superheating zone remains substantially constant until superheating zone outflow to the separating zone is cut ofl.

6. The method of starting a power plant in which, during normal operation, a vaporizable fluid is successively passed through a fluid heating zone, a first superheating zone and a second superheating zone to a vapor turbine, said method comprising passing a vaporizable fluid at about full load pressure through the heating zone and the first superheating zone in indirect heat transfer relation with heating gases and at a rate substantially less than full load flow,

passing the fluid so heated to a separating zone while reducing its pressure to cause part of the fluid to flash into vapor,

recirculating fluid through the heating zone, first superheating zone and separating zone to the heating zone to increase the enthalpy of the first superheating zone outflow to a predetermined value, while increasing separating zone fluid pressure to a predetermined value substantially less than turbine full throttle pressure,

passing vapor formed in the separating zone through the second superheating zone to the turbine in indirect heat transfer relation with heating gases to warm and roll the turbine,

maintaining pressure in the heating zone and the first superheating zone at about turbine full throttle pressure, while regulating flow of fluid from the first superheating zone to the separating zone so that the enthalpy of the fluid discharging from the first superheating zone is maintained at about said predetermined value, and

gradually establishing equilibrium of fluid pressures in the first and second superheatingzones by passing first superheating zone outflow through the second superheating zone to the turbine at a rate providing gradual increase in flow to the turbine, while discontinuing vapor flow from the separating zone to the second superheating zone and while decreasing flow from the first superheating zone to the separating zone so that flow through the first superheating zone remains substantially constant until first superheating zone outflow to the separating zone is cutoff.

7. The method of starting a power plant in which,

during normal operation, a vaporizable fluid is successively passed through a fluid heating zone, a first superheating zone and a second superheating zone to a vapor turbine, said method comprising passing a vaporizable fluid at about turbine full throttle pressure through the heating zone in indirect heat transfer relation with heating gases and at a predetermined rate substantially less than full load flow,

passing a portion of the heating zone outflow to a separating zone while reducing its pressure to cause part of the fluid to flash into vapor,

passing the remainder of the heating zone outflow through the first superheating zone in indirect heat transfer relation with the heating gases,

passing the first superheating zone outflow to the separating zone while reducing its pressure to cause part of the fluid to flash into vapor,

recirculating fluid through the heating zone, first superheating zone and separating zone to the heating zone to increase the enthalpy of the first superheating zone outflow to a predetermined value, while increasing separating zone fluid pressure to a predetermined value substantially less than turbine full throttle pressure,

passing vapor formed in the separating zone through the second superheating zone to the turbine in indirect heat transfer relation with heating gases to warm and roll the turbine,

maintaining pressure in the heating zone and the first superheating zone at about turbine full throttle'pressure, while regulating flow of fluid from the first superheating zone to the separating zone so that the enthalpy of the fluid discharging from the first superheating zone is maintained at about said predetermined value, said enthalpy value being about the same as the enthalpy of the separating zone vapor outflow, and

gradually establishing equilibrium of fluid pressures in the first and second superheating zones by passing first superheating zone outflow through the second superheating zone to the turbine at a rate providing gradual increase in flow of the turbine, while discontinuing vapor flow from the separating zone to the second superheating zone and while decreasing flow from the first superheating zone to the separatmg zone. 8. The method of starting a power plant in which, durlng normal operation, a vaporizable fluid is successively passed through a fluid heating zone, a first superheating zone and a second superheating zone to a vapor turbine, said method comprising passing a vaporizable fluid at about turbine full throttle pressure through the heating zone in indirect heat transfer relation with heating gases and at a predetermined rate substantially less than full load flow, passing a portion of the heating zone outflow to a separating zone while reducing its pressure to cause part of the fluid to flash into vapor, passing the remainder of the heating zone outflow through the first superheating zone in indirect heat transfer relation with the heating gases, passing the first superheating zone outflow to the separating zone while reducing its pressure to cause part of the fluid to flash into vapor, recirculating fluid through the heating Zone, first superheating zone and separating zone to the heating zone to increase the enthalpy of the first superheating zone outflow to a predetermined value, while increasing separating zone fluid pressure to a predetermined value substantially less than turbine full throttle pressure, passing-vapor formed in the separating zone through the second superheating zone to the turbine in indirect heat transfer relation with heating gases to warm and roll the turbine, maintaining pressure in the heating zone and the first superheating zone at about turbine full throttle pressure, while regulating flow of fluid from the first superheating zone to the separating zone so that the enthalpy of the fluid discharging from the first superheating zone is maintained at about said predetermined value, said enthalpy value being about the same as the enthalpy of the separating zone vapor outflow, and gradually establishing equilibrium of fluid pressures in the first and second superheating zones by passing first superheating zone outflow through the second superheating zone to the turbine at a rate providing gradual increase in flow to the turbine, while discontinuing vapor flow from the separating zone to the second superheating zone and while decreasing flow from the first superheating zone to the separating zone so that flow through the first superheating zone remains substantially constant until first superheating zone outflow to the separating zone is cut oil. 9. In a power plant in which, during normal operation,

a vaporizable fluid is successively passed through a fluid heating section, a first superheater and a secondsuperheater to a vapor turbine, a start-up system comprising means for generating heating gases,

means for passing vaporizable fluid at about turbine full throttle pressure through the heating section in indirect heat transfer relation with the heating gases and at a predetermined rate substantially less than full load flow,

means for passing a portion of the heating section outflow to a separator while reducing its pressure to cause part of the fluid to flash into vapor,

means for passing the remainder of the heating section outflow through the first superheater in indirect heat transfer relation with the heating gases,

means for passing the first superheater outflow to the separator while reducing its pressure to cause part of the fluid to flash into vapor,

means for recirculating fluid through the heating section, first superheater and separator to the heating section to increase the enthalpy of the first superheater outflow to a predetermined value,

means for passing vapor formed in the separator through the second superheater to the turbine in indirect heat transfer relation with the heating gases to warm and roll the turbine,

means for maintaining pressure in the heating section and the first superheater at about turbine full throttle pressure,

means for maintaining the enthalpy of the fluid discharging from the first superheater at about said predetermined value until equilibrium of pressures between the first and second superheaters is established, and

means for gradually establishing equilibrium of fluid pressures in the first and second superheaters.

10. In a power plant in which, during normal operation, a vaporizable fluid is successively passed through a first heating section and a second heating section to a vapor turbine, a start-up system comprising means for generating heating gases,

means for passing vaporizable fluid at about turbine full throttle pressure through the first heating section in indirect heat transfer relation with the heating gases and at a predetermined rate substantially less than full load flow,

means for passing a portion of the first heating section outflow to -a separator while reducing its pressure to cause part of the fluid to flash into vapor,

means for passing the remainder of the first heating section outflow through the second heating section in indirect heat transfer relation with the heating gases,

means for passing the second heating section outflow to the separator while reducing its pressure to cause part of the fluid to flash into vapor,

means for recirculating fluid through the first heating section, second heating section and separator to the first heating section to increase the enthalpy of the second heating section outflow to a predetermined value,

means for passing vapor formed in the separator to the turbine to warm and roll the turbine,

means for maintaining pressure in the first heating section and the second heating section at about turbine full throttle pressure,

means for maintaining the enthalpy of the fluid discharging from the second heating section at about said predetermined value until equilibrium of pressures between the second heating section and the turbine is established, and

means for gradually establishing equilibrium of fluid pressures between the second heating section and the turbine. 11. In a power plant in which, during normal operation, a vaporizable fluid is successively passed through a fluid heating section, a first superheater and a second superheater to a vapor turbine, a start-up system comprising means for generating heating gases, means for passing vaporizable fluid at about turbine full throttle pressure through the heating section in indirect heat transfer relation with the heating gases and at a predetermined rate substantially less than full load flow,

means for passing a portion of the heating section outflow to a separator while reducing its pressure to cause part of the fluid to flash into vapor,

means for passing the remainder of the heating section outflow through the first superheater in indirect heat transfer relation with the heating gases,

means for passing the first superheater outflow to the separator while reducing its pressure to cause part of the fluid to flash into vapor,

means for maintaining pressure in the heating section and the first superheater at about turbine full throttle pressure,

means for passing vapor formed in the separator through the second superheater to the turbine in indirect heat transfer relation with the heating gases to warm and roll the turbine,

means for increasing the enthalpy of the fluid discharging from the first superheater to a value about the same as the enthalpy of the separator vapor outflow and for maintaining this value until equilibrium of pressures between the first and second superheaters is established, and

5 means for gradually establishing equilibrium of fluid pressures in the first and second superheaters.

No references cited.

MARTIN P. SCHWADRON, Primary Examiner.

ROBERT R. BUNEVICH, Examiner. 

1. THE METHOD OF STARTING A POWER PLANT IN WHICH, DURING NORMAL OPERATION, A VAPORIZABLE FLUID IS SUCCESSIVELY PASSED THROUGH A FIRST HEATING ZONE AND A SECOND HEATING ZONE TO A VAPOR TURBINE, SAID METHOD COMPRISING PASSING VAPORIZABLE FLUID AT ABOUT TURBINE FULL THROTTLE PRESSURE THROUGH THE FIRST HEATING ZONE IN DIRECT HEAT TRANSFER RELATION WITH HEATING GASES AND AT A PREDETERMINED RATE SUBSTANTIALLY LESS THAN FULL LOAD FLOW, PASSING A SUBSTANTIAL PORTION OF THE FIRST HEATING ZONE OUTFLOW THROUGH THE SECOND HEATING ZONE IN INDIRECT HEAT TRANSFER RELATION WITH THE HEATING GASES, PASSING SECOND HEATING ZONE OUTFLOW TO THE SEPARATING ZONE WHILE REDUCING ITS PRESSURE TO CAUSE PART OF THE FLUID TO FLASH INTO VAPOR, RECIRCULATING FLUID THROUGH THE FIRST HEATING ZONE, SECOND HEATING ZONE AND SEPARATING ZONE TO THE FIRST HEATING ZONE TO INCREASE THE ENTHALPY OF THE SECOND HEATING ZONE OUTFLOW TO A PREDETERMINED VALUE, WHILE INCREASING SEPARATING ZONE FLUID PRESSURE TO A PREDETERMINED VALUE SUBSTANTIALLY LESS THAN TURBINE FULL THROTTLE PRESSURE, PASSING VAPOR FORMED IN THE SEPARATING ZERO TO THE TURBINE TO WARM AND ROLL THE TURBINE, MAINTAINING PRESSURE IN THE FIRST HEATING ZONE AND THE SECOND HEATING ZONE AT ABOUT TURBINE FULL THROTTLE PRESSURE, WHILE REGULATING FLOW OF FLUID FROM THE SECOND HEATING ZONE TO THE SEPARATING ZONE SO THAT THE ENTHALPY OF THE FLUID DISCHARGING FROM THE SECOND HEATING ZONE IS MAINTAINED AT ABOUT SAID PREDETERMINED VALUE, GRADUALLY ESTABLISHING EQUILIBRIUM OF FLUID PRESSURES BETWEEN THE SECOND HEATING ZONE AND THE TURBINE BY PASSING SECOND HEATING ZONE OUTFLOW TO THE TURBINE AT A RATE PROVIDING GRADUAL INCREASE IN FLOW TO THE TURBINE, WHILE DISCONTINUING VAPOR FLOW FROM THE SEPARATING ZONE TO THE TURBINE, AND DISCONTINUING FLUID FLOW FROM THE SECOND HEATING ZONE TO THE SEPARATING ZONE. 